1. Field of the Invention
The present invention generally relates to semiconductor devices, and in particular, to a method of forming a pattern using a chemical amplification resist.
2. Description of the Related Art
With miniaturization of semiconductor devices, the exposing wavelength used in the lithography process has become very small in recent advanced semiconductor fabrication processes. Particularly, the use of deep ultra-violet (DUV) radiation source such as KrF or ArF excimer lasers is spreading rapidly.
When such DUV is used as the exposing radiation, the resist is more easily influenced by the multiple-interference effect caused by optical reflection at the substrate surface as compared to the case of using conventional i-line radiation. Therefore, in many cases, an anti-reflection coating commonly referred to as BARC (Bottom Anti-Reflection Coating) is provided under the resist so as to reduce the influence of the reflection and multiple-interference on the resist.
Furthermore, in many cases of high-resolution exposure that uses DUV, an optical system having a large numerical aperture is used for enhancing the resolution. When an optical system of large numerical aperture is used, on the other hand, it becomes difficult to secure sufficient focal depth at the time of the exposure as is well known in the art, while this makes it necessary to use a resist of reduced thickness.
In order to solve the problem of etching resistance of the resist, which is caused at the time of using such a thin resist, it is practiced to provide a film having a composition different from the relevant resist under the resist, and etching is performed while using such a film as the mask.
Thus, the use of multi-layered resist is now becoming an established effective exposure process in the art of high-resolution photolithography that uses DUV.
Moreover, it is now common to use a chemical amplification resist in the art of high-resolution photolithography for achieving the high resolution and high photosensitivity for the resist. A chemical amplification resist is a resist containing a photoacid generator and the photoacid generator releases a photoacid upon exposure. In the chemical amplification resist, the release of photoacid is accelerated by the catalytic action of the photoacid, and the resist is changed rapidly to have an alkali-soluble molecular structure after the exposure. This type of resist is called positive-type Further, there is also a chemical amplification resist of negative type.
In the case of using such a chemical amplification resist, it should be noted that the size and resolution of the resist patterns may change depending on the degree of diffusion of the photoacid. For example, the exposed patterns may be influenced by the existence of acid or base at the interface between the resist and the underlying film.
In the case when a BARC is formed under the resist as mentioned above, for example, there may be a case in which the resist pattern shows a defective shape called skirting in which the resist remains at the bottom part of the pattern from which the resist should be removed. This phenomenon occurs when the photoacid released by the photoacid generator of the resist is deactivated by a component contained in the BARC.
Hereinafter, the process using the multi-layered resist will be considered for the case of a three-layer process. It should be noted that the three-layer process is a method in which an insulating film such as an SOG film is formed under the resist and a polymer film such as a novolac resin film and the like is formed further under the SOG film. In the three-layer process, the desired patterning is achieved in a layer located underneath the polymer film.
Thus, a pattern formation is conducted at first by etching the SOG film while using the chemical amplification resist pattern formed thereon as a mask, and the polymer film is then etched while using the patterned SOG film as the mask. Further, the pattern thus formed in the polymer film is transferred to the layer underneath the polymer film by a dry etching process or the like. This layer may be an insulating film such as a silicon oxide film.
In this three-layer process, the polymer film functions as the BARC or anti-reflection as noted before. This three-layer process has an advantageous feature of enabling the use of thin resist at the top and is used extensively these days.
In the above-noted three-layer process, on the other hand, there can be a case in which the photoacid released from the chemical amplification resist at the top of the three-layer structure is deactivated by the components in the SOG film provided right underneath the resist, similarly to the case in which the BARC is used underneath the chemical amplification resist. Thereby a similar problem of skirting may occur.
In order to deal with this problem, it is practiced currently to add a photoacid generator to the BARC film or to the SOG film so as to compensate for the deactivated acid.
When a photoacid generator is added to the film right underneath the resist, the acid concentration at the interface between the BARC and the resist, or at the interface between the SOG and the resist, increases, wherein the excessive acid thus introduced removes the protective group of the resist and makes the resist alkali-soluble. Thus, such an approach gives rise to another problem of formation of cut-in at the resist interface. In the case of a negative resist, such an approach causes the problem of skirting or scum residue formation at the bottom part of the resist.
In the case of using an SOG film, in particular, there arises a further problem in that the adhesion to the resist may be deteriorated as a result of addition of such extraneous photoacid generator. Further, because of difficulty in distributing the photoacid generator uniformly on the above SOG film, there arises the problem in that the patterns may vary depending on the location of the wafer surface. Furthermore, the efficiency of the photoacid formation changes depending on the exposure dose, which in turn is changed depending on the covering ratio of the reticle used for the exposure. Thus it is very difficult to control the amount of the photoacid generator appropriately. Further, there has been a problem in that it takes time, in the event the resist is changed, of adding and adjusting the photoacid generator to the SOG film.
FIG. 1A and FIG. 1B show an example of inappropriate or uncontrolled pattern formation obtained for the case of using the conventional three-layered resist process mentioned above.
Referring to FIGS. 1A and 1B, the patterns A and B are separated from the substrate to be processed and it can be seen that the pattern A is toppled and lying on the substrate. It is believed that this phenomenon is caused because of the existence of excessive acid at the interface between the resist and the underlying film.